Random engagement roller chain sprocket having improved noise characteristics

ABSTRACT

A unidirectional roller chain sprocket for use primarily in automotive engine chain drive applications which incorporates an asymmetrical tooth for improved noise reduction. The sprocket includes a first plurality of sprocket teeth each having a first engaging flank with a first contact point at which a roller contacts the first engaging flank, and a second plurality of sprocket teeth each having a second engaging flank with a second contact point at which a roller contacts the second engaging flank. The first engaging flanks include a flank flat which facilitates spacing the first contact point on the first engaging flank relative to the second contact point on the second engaging flank to effect a time delay between an initial roller to first sprocket tooth contact and an initial roller to second sprocket tooth contact. The flank flat is tangent to an engaging flank radius and a first root radius. The asymmetrical tooth profile may also incorporate one or more inclined root surfaces which provide tooth space clearance for maintaining the chain rollers in hard contact with the root surface in the sprocket wrap. The asymmetrical tooth profile may also incorporate added pitch mismatch wherein the sprocket chordal pitch is less than the chain link pitch to facilitate a &#34;staged&#34; roller-tooth contact as a roller moves into full mesh from an initial tangential impact at the flank flat.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is continuation of application Ser. No. 08/900,661,filed Jul. 25, 1997, now U.S. Pat. No. 5,976,045, which claims priorityfrom U.S. Provisional Patent Application Serial Nos. 60/022,321, filedJul. 25, 1996 and 60/032,379, filed Dec. 19, 1996.

BACKGROUND OF THE INVENTION

The present invention relates to the automotive timing chain art. Itfinds particular application in conjunction with a unidirectional rollerchain sprocket for use in automotive camshaft drive applications andwill be described with particular reference thereto. However, thepresent invention may also find application in conjunction with othertypes of chain drive systems and applications where reducing the noiselevels associated with chain drives is desired.

Roller chain sprockets for use in camshaft drives of automotive enginesare typically manufactured according to ISO (International Organizationfor Standardization) standard 606:1994(E). The ISO-606 standardspecifies requirements for short-pitch precision roller chains andassociated chain wheels or sprockets.

FIG. 1 illustrates a symmetrical tooth space form for an ISO-606compliant sprocket. The tooth space has a continuous fillet or rootradius R_(i) extending from one tooth flank (i.e., side) to the adjacenttooth flank as defined by the roller seating angle α. The flank radiusR_(f) is tangent to the roller seating radius R_(i) at the tangencypoint TP. A chain with a link pitch P has rollers of diameter D₁ incontact with the tooth spaces. The ISO sprocket has a chordal pitch alsoof length P, a root diameter D₂, and Z number of teeth. The pitch circlediameter PD, tip or outside diameter OD, and tooth angle A (equal to360°/Z) further define the ISO-606 compliant sprocket. The maximum andminimum roller seating angle α is defined as:

    α.sub.max =(140°-90°)/Z and α.sub.min =(120°-90°)/Z

With reference to FIG. 2, an exemplary ISO-606 compliant roller chaindrive system 10 rotates in a clockwise direction as shown by arrow 11.The chain drive system 10 includes a drive sprocket 12, a drivensprocket 14 and a roller chain 16 having a number of rollers 18. Thesprockets 12, 14, and chain 16 each generally comply with the ISO-606standard.

The roller chain 16 engages and wraps about sprockets 12 and 14 and hastwo spans extending between the sprockets, slack strand 20 and tautstrand 22. The roller chain 16 is under tension as shown by arrows 24.The taut strand 22 may be guided from the driven sprocket 14 to thedrive sprocket 12 with a chain guide 26. A first roller 28 is shown atthe onset of meshing at a 12 o'clock position on the drive sprocket 12.A second roller 30 is adjacent to the first roller 28 and is the nextroller to mesh with the drive sprocket 12.

Chain drive systems have several components of undesirable noise. Amajor source of roller chain drive noise is the sound generated as aroller leaves the span and collides with the sprocket during meshing.The resultant impact noise is repeated with a frequency generally equalto that of the frequency of the chain meshing with the sprocket. Theloudness of the impact noise is a function of the impact energy (E_(A))occuring during the meshing process. The magnitude of the impact energyis related to engine speed, chain mass, and the impact velocity betweenthe chain and the sprocket at the onset of meshing. The impact velocityis affected by the chain-sprocket engagement geometry, of which anengaging flank pressure angle γ is a factor, where: ##EQU1##

The impact energy equation presumes the chain drive kinematics willconform generally to a quasistatic analytical model and that theroller-sprocket driving contact will occur at a tangency point TP (FIG.3) for the flank and root radii as the sprocket collects a roller fromthe span.

As shown in FIG. 3, the pressure angle γ is defined as the angle betweena line A extending from the center of the engaging roller 28, when it iscontacting the engaging tooth flank at the tangency point TP, throughthe center of the flank radius R_(f), and a line B connecting thecenters of the fully seated roller 28, when it is seated on rootdiameter D₂, and the center of the next meshing roller 30 as if it werealso seated on root diameter D₂ in its engaging tooth space. The rollerseating angles α and pressure angles γ listed in FIG. 27 are calculatedfrom the equations defined above. It should be appreciated that γ is aminimum when α is a maximum. The exemplary 18-tooth ISO-606 compliantsprocket 12 of FIG. 3 will have a pressure angle γ in the range of 12.5°to 22.5° as listed in the table of FIG. 27.

FIG. 3 also shows the engagement path (phantom rollers) and the drivingcontact position of roller 28 (solid) as the drive sprocket 12 rotatesin the direction of arrow 11. It is assumed that the chain drivekinematics generally conform to, and can be represented by aquasi-static analytical model, and that the drive contact occurs at thetangency point TP. FIG. 3 depicts the theoretical case with chain roller27 seated on root diameter D₂ of a maximum material sprocket with bothchain pitch and sprocket chordal pitch equal to theoretical pitch P. Forthis theoretical case, the noise occurring at the onset of rollerengagement has a radial component F_(IR) as a result of roller 28colliding with the root surface R_(i) and a tangential component F_(IT)generated as the same roller 28 collides with the engaging tooth flankat point TP as the roller moves into driving contact. It is believedthat the radial impact occurs first, with the tangential impactfollowing nearly simultaneously. Roller impact velocity V_(A) is shownto act through, and is substantially normal to, engaging flank tangentpoint TP with roller 28 in driving contact at point TP.

The impact energy (E_(A)) equation accounts only for a tangential rollerimpact during meshing. The actual roller engagement, presumed to havetangential and radial impacts (occurring in any order), would thereforeseem to be at variance with the impact energy (E_(A)) equation. Theapplication of this quasi-static model, which is beneficially used as adirectional tool, permits an analysis of those features that may bemodified to reduce the impact energy occurring during the tangentialroller-sprocket collision at the onset of meshing. The radial collisionduring meshing, and its effect on the meshing noise levels, can beevaluated apart from the impact energy (E_(A)) equation.

Under actual conditions as a result of feature dimensional tolerances,there will normally be a pitch mismatch between the chain and sprocket,with increased mismatch as the components wear in use. This pitchmismatch serves to move the point of meshing impact, with the radialcollision still occurring at the root surface R_(i) but not necessarilyat D₂. The tangential collision will normally be in the proximity ofpoint TP, but this contact could take place high up on the engaging sideof root radius R_(i) or even radially outward from point TP on theengaging flank radius R_(f) as a function of the actual chain-sprocketpitch mismatch.

Reducing the engaging flank pressure angle γ reduces the meshing noiselevels associated with roller chain drives, as predicted by the impactenergy (E_(A)) equation set forth above. It is feasible but notrecommended to reduce the pressure angle γ while maintaining asymmetrical tooth profile, which could be accomplished by simplyincreasing the roller seating angle α, effectively decreasing thepressure angle for both flanks. This profile as described requires thata worn chain would, as the roller travels around a sprocket wrap(discussed below), interface with a much steeper incline and the rollerswould necessarily ride higher up on the coast flank prior to leaving thewrap.

Another source of chain drive noise is the broadband mechanical noisegenerated in part by shaft torsional vibrations and slight dimensionalinaccuracies between the chain and the sprockets. Contributing to agreater extent to the broadband mechanical noise level is theintermittent or vibrating contact that occurs between an unloaded rollerand the sprocket as the roller travels around the sprocket wrap. Inparticular, ordinary chain drive system wear comprises sprocket toothface wear and chain wear. The chain wear is caused by bearing wear inthe chain joints and can be characterized as pitch elongation. It isbelieved that a worn chain meshing with an ISO standard sprocket willhave only one roller in driving contact and loaded at a maximum loadingcondition.

With reference again to FIG. 2, driving contact at maximum loadingoccurs as a roller enters a drive sprocket wrap 32 at engagement.Engaging roller 28 is shown in driving contact and loaded at a maximumloading. The loading on roller 28 is primarily meshing impact loadingand the chain tension loading. The next several rollers in the wrap 32forward of roller 28 share in the chain tension loading, but at aprogressively decreasing rate. The loading of roller 28 (and to a lesserextent for the next several rollers in the wrap) serves to maintain theroller in solid or hard contact with the sprocket root surface 34.

A roller 36 is the last roller in the drive sprocket wrap 32 prior toentering the slack strand 20. Roller 36 is also in hard contact withdrive sprocket 12, but at some point higher up (e.g., radiallyoutwardly) on the root surface 34. With the exception of rollers 28 and36, and the several rollers forward of roller 28 that share the chaintension loading, the remaining rollers in the drive sprocket wrap 32 arenot in hard contact with the sprocket root surface 34, and are thereforefree to vibrate against the sprocket root surfaces as they travel aroundthe wrap, thereby contributing to the generation of unwanted broadbandmechanical noise.

A roller 38 is the last roller in a sprocket wrap 40 of the drivensprocket 14 before entering the taut strand 22. The roller 38 is indriving contact with the sprocket 14. As with the roller 36 in the drivesprocket wrap 32, a roller 42 in the sprocket wrap 40 is in hard contactwith a root radius 44 of driven sprocket 14, but generally not at theroot diameter.

It is known that providing pitch line clearance (PLC) between sprocketteeth promotes hard contact between the chain rollers and sprocket inthe sprocket wrap as the roller chain wears. The amount of pitch lineclearance added to the tooth space defines a length of a short arc thatis centered in the tooth space and forms a segment of the root diameterD₂. The root fillet radius R_(i) is tangent to the flank radius R_(F)and the root diameter arc segment. The tooth profile is stillsymmetrical, but R_(i) is no longer a continuous fillet radius from oneflank radius to the adjacent flank radius. This has the effect ofreducing the broadband mechanical noise component of a chain drivesystem. However, adding pitch line clearance between sprocket teeth doesnot reduce chain drive noise caused by the roller-sprocket collision atengagement.

Chordal action, or chordal rise and fall, is another important factoraffecting the operating smoothness and noise levels of a chain drive,particularly at high speeds. Chordal action occurs as the chain entersthe sprocket from the free span during meshing and it can cause amovement of the free chain in a direction perpendicular to the chaintravel but in the same plane as the chain and sprockets. This chainmotion resulting from chordal action will contribute an objectionablenoise level component to the meshing noise levels, so it is thereforebeneficial to reduce chordal action inherent in a roller chain drive.

FIGS. 4a and 4b illustrate the chordal action for an 18-tooth ISO-606compliant sprocket having a chordal pitch of 9.525 mm. Chordal rise 45may conventionally be defined as the displacement of the chaincenterline as the sprocket rotates through an angle A/2, where:

    Chordal rise=r.sub.p -r.sub.c =r.sub.p [1-cos (180°/Z)]

where r_(c) is the chordal radius, or the distance from the sprocketcenter to a pitch chord of length P; r_(p) is the actual theoreticalpitch radius; and Z is the number of sprocket teeth.

It is known that a short pitch chain provides reduced chordal actioncompared to a longer pitch chain having a similar pitch radius. FIGS. 4aand 4b show only the drive sprocket 12 and assume driven sprocket (notshown) also having 18-teeth and in phase with the drive sprocket shown.In other words, at T=0, (see FIG. 4a) both sprockets, substantiallyidentical, will have a tooth center at the 12 o'clock position.Accordingly, this chain drive arrangement under quasi-static conditionswill have a top or taut strand that will move up and down in a uniformmanner a distance equal to that of the chordal rise. At T=0, roller 46is at the onset of meshing, with chordal pitch P horizontal and in linewith taut strand 22. At T=0+(A/2) (see FIG. 4b), roller 46 has moved tothe 12 o'clock position.

For many chain drives, the drive and driven sprockets will be ofdifferent sizes and will not necessarily be in phase. The chain guide 26(FIG. 2) has the primary purpose to control chain strand vibration inthe taut span. The geometry of the guide-chain chain interface alsodefines the length of unsupported chain over which chordal rise and fallis allowed to articulate. FIG. 5 is an enlarged view of FIG. 2 showingthe first roller 28 at the onset of engagement and a second roller 30 asthe next roller about to mesh with sprocket 12. In this example, thechain guide 26 controls and guides the taut strand 22 except for five(5) unsupported link pitches extending between the chain guide 26 andthe engaging roller 28. The taut strand 22 is horizontal when the roller28 is at the 12 o'clock position.

With reference to FIGS. 6 and 7, the drive sprocket 12 is rotated in aclockwise direction to advance roller 28 to a new angular position(A/2)+ω, determined by the instant of sprocket engagement of roller 30.Roller 28 is considered to be seated and in hard contact with the rootsurface at D₂ at the onset of meshing of roller 30. As shown in FIG. 6,a straight line is assumed for the chain span from roller 28 to a chainpin center 48, about which the unsupported span from pin 48 to engagingroller 30 is considered to rotate. The location and chain-interfacingcontour of the chain guide 26 will determine the number of free spanpitches about which articulation will take place as a result of thechordal rise and fall during the roller meshing process.

As best seen in FIG. 7, assuming that rollers 28 and 30 are in hardcontact with the sprocket root surfaces at D₂, the chordal rise is theperpendicular displacement of the center of roller 30 (located on thepitch diameter PD) from the taut span 22 path as it moves from itsinitial meshing position shown to the 12 o'clock position.

Accordingly, it is desirable to develop a new and improved roller chaindrive system which meets the above-stated needs and overcomes theforegoing disadvantages and others while providing better and moreadvantageous results.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a roller chainsprocket is disclosed. The roller chain sprocket includes a firstplurality of sprocket teeth each having a first tooth profile includinga first engaging flank with a first contact point at which a rollerinitially contacts the first engaging flank, a second plurality ofsprocket teeth each having a second tooth profile including a secondengaging flank with a second contact point at which a roller initiallycontacts the second engaging flank; and the first tooth profileincluding a flank flat on the first engaging flank which facilitatesspacing the first contact point on the first engaging flank relative tothe second contact point on the second engaging flank to alter aninitial roller-to-first engaging flank contact relative to an initialroller-to-second engaging flank contact. Thus, the flank flatfacilitates altering the meshing contact from the first profile to thesecond. That is, while the frequency of the contacts (at constant rpm)will be substantially identical for each tooth profile, the profiledifferences serve to shift the initial contact phasing, thereby alteringthe rhythm of the initial contacts from one profile to the other.

In accordance with another aspect of the present invention, aunidirectional roller chain drive system is disclosed. Theunidirectional roller chain drive system includes a driving sprockethaving sprocket teeth with engaging flanks and coast flanks, theengaging flanks cooperating with the coast flanks of adjacent teeth todefine asymmetrical tooth spaces between the sprocket teeth, a drivensprocket having sprocket teeth with engaging flanks and coast flanks,the engaging flanks cooperating with the coast flanks of adjacent teethto define asymmetrical tooth spaces between the sprocket teeth, a rollerchain having rollers in engaging contact with the driving sprocket andthe driven sprocket, and at least one of the driving sprocket and thedriven sprocket including a first plurality of sprocket teeth eachhaving a first engaging flank with a first contact point at which aroller initially contacts the first engaging flank, a second pluralityof sprocket teeth each having a second engaging flank with a secondcontact point at which a roller initially contacts the second engagingflank, and the first engaging flank including a flank flat whichfacilitates phasing a frequency of initial roller-to-first engagingflank contacts relative to initial roller-to-second engaging flankcontacts to alter the rhythm of the initial roller-to-first engagingflank and the roller-to-second engaging flank contacts. As with thefirst aspect of the invention, the flank flat facilitates altering themeshing contact from the first profile to the second.

In accordance with yet another aspect of the present invention, a rollerchain sprocket is disclosed. The roller chain sprocket includes a firstplurality of sprocket teeth each having a first engaging flank with afirst contact point at which a roller initially contacts the firstengaging flank, a second plurality of sprocket teeth each having asecond engaging flank with a second contact point at which a rollerinitially contacts the second engaging flank, and the first engagingflank including a flank flat which facilitates phasing a frequency ofinitial roller-to-first engaging flank contacts relative to initialroller-to-second engaging flank contacts to alter the rhythm of theinitial roller-to-first engaging flank and the roller-to-second engagingflank contacts.

One advantage of the present invention is the provision of a rollerchain sprocket which incorporates a flank flat on an engaging toothsurface which facilitates altering a meshing contact from a first toothprofile to a second tooth profile.

Another advantage of the present invention is the provision of a rollerchain sprocket which incorporates a flank flat on an engaging toothsurface which effects a time delay between an initial roller-to-firstsprocket tooth profile contact and an initial roller-to-second sprockettooth profile contact.

Another advantage of the present invention is the provision of a rollerchain sprocket which incorporates a flank flat on an engaging toothsurface of a first tooth profile which facilitates phasing a frequencyof initial roller-to-engaging flank contacts of the first tooth profilerelative to initial roller-to-engaging flank contacts of a second toothprofile to alter the rhythm of the initial roller-to-first engagingflank and the roller-to-second engaging flank contacts.

Another advantage of the present invention is the provision of a rollerchain sprocket which incorporates added pitch mismatch between thesprocket and roller chain to facilitate a "staged" roller-to-sprocketimpact.

Still another advantage of the present invention is the provision of aroller chain sprocket which incorporates an inclined root surface on anengaging flank, a coast flank, or both an engaging flank and a coastflank to provide tooth space clearance.

Yet another advantage of the present invention is the provision of aroller chain sprocket which minimizes impact noise generated by aroller-sprocket collision during meshing.

A further advantage of the present invention is the provision of aroller chain sprocket which minimizes broadband mechanical noisegenerated by unloaded rollers in a sprocket wrap.

A still further advantage of the present invention is the provision of aroller chain sprocket which provides a "staged" roller impact wherein atangential impact at full mesh occurs first followed by a radial impact.

Yet a further advantage of the present invention is the provision of aroller chain sprocket which spreads roller engagement over a significanttime interval to provide for a more gradual load transfer, therebyminimizing roller-sprocket impact and the inherent noise generatedtherefrom.

Yet a further advantage of the present invention is the provision of aroller chain sprocket having two sets of sprocket teeth incorporatingdifferent tooth profiles which cooperate to reduce chain drive systemnoise levels below a noise level which either tooth profile used alonewould produce.

Further advantages of the present invention will become apparent tothose of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 illustrates a symmetrical tooth space form for an ISO-606compliant roller chain sprocket;

FIG. 2 is an exemplary roller chain drive system having a ISO-606compliant drive sprocket, driven sprocket, and roller chain;

FIG. 3 shows an engagement path (phantom) and a roller (solid) in adriving position as an ISO-606 compliant drive sprocket rotates in aclockwise direction;

FIG. 4a shows a roller at the onset of meshing with an exemplary 18tooth sprocket;

FIG. 4b shows the drive sprocket of FIG. 4a rotated in a clockwisedirection until the roller is at a 12 o'clock position.

FIG. 5 is an enlarged view of the drive sprocket of FIG. 2 with a rollerfully seated in a tooth space and a second roller about to mesh with thedrive sprocket;

FIG. 6 shows the drive sprocket of FIG. 5 rotated in a clockwisedirection until the second roller initially contacts the drive sprocket;

FIG. 7 is an enlarged view of FIG. 6 showing that the second rollerinitially contacts a root surface (i.e., radial impact) of the drivesprocket, under theoretical conditions;

FIG. 8 illustrates a roller chain drive system having a roller chaindrive sprocket and driven sprocket which incorporate the features of thepresent invention therein;

FIG. 9 illustrates the roller chain drive sprocket of FIG. 8 with anasymmetrical tooth space form in accordance with a first embodiment ofthe present invention;

FIG. 10 is an enlarged view of the asymmetrical tooth space form of FIG.9 showing a roller in two-point contact with the sprocket;

FIG. 11 shows an engagement path (phantom) and the full mesh position(solid) of a roller as the drive sprocket of FIG. 8 rotates in aclockwise direction;

FIG. 12 is an enlarged view of the drive sprocket of FIG. 8 with aroller fully seated in a tooth space and a second roller as the nextroller to be collected from a taut span of the roller chain;

FIG. 13 shows the drive sprocket of FIG. 12 rotated in a clockwisedirection until the second roller initially contacts the drive sprocket;

FIG. 14 is an enlarged view of FIG. 13 showing the first roller intwo-point contact and second roller at initial tangential contact withthe drive sprocket;

FIG. 14a illustrates the progression of a first and a second roller asthe roller chain drive sprocket of FIG. 8 is rotated in a clockwisedirection;

FIG. 14b is an enlarged view of the drive sprocket of FIG. 14 rotated ina clockwise direction to advance the second roller to the instant offull mesh at a 12 o'clock position;

FIG. 15 illustrates a roller chain drive sprocket with an asymmetricaltooth space form in accordance with a second embodiment of the presentinvention;

FIG. 16 is an enlarged view of FIG. 8, showing the contact progressionas the rollers travel around the drive sprocket wrap;

FIG. 17 is an enlarged view of a roller exiting a sprocket wrap of thesprocket of FIG. 8;

FIG. 18 illustrates a roller chain sprocket with an asymmetrical toothspace form in accordance with a third embodiment of the presentinvention;

FIG. 19 illustrates a roller chain sprocket with an asymmetrical toothspace form in accordance with a fourth embodiment of the presentinvention;

FIG. 20 is a side view of an exemplary random-engagement roller chainsprocket which incorporates the features of the present inventiontherein;

FIG. 21 illustrates the random-engagement roller chain sprocket of FIG.20 with an additional asymmetrical tooth space form in accordance withthe present invention;

FIG. 22 illustrates the asymmetrical tooth space form of FIG. 9 overlaidwith one embodiment of the asymmetrical tooth space form of FIG. 21;

FIG. 23 illustrates the asymmetrical tooth space form of FIG. 9overlayed with a second embodiment of the asymmetrical tooth space formof FIG. 21;

FIG. 24 illustrates the progression of a first and a second roller asthe random-engagement roller chain sprocket of FIG. 20 is rotated in aclockwise direction;

FIG. 25 illustrates the sprocket of FIG. 23 with a first roller intwo-point contact, a second roller at initial tangential contact, and athird roller about to mesh with the drive sprocket;

FIG. 26 illustrates the sprocket of FIG. 25 rotated in a clockwisedirection until the instant of initial tangential contact between thethird roller and the roller chain drive sprocket;

FIG. 27 is a table listing roller seating angles (α) and pressure angles(γ) for a number of different ISO-606 compliant roller chain sprockets;and

FIG. 28 is a table listing the maximum (β) angles and the correspondingminimum pressure angles (γ) for three different asymmetrical tooth spaceprofiles (1-3) of varying sprocket sizes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to FIG. 8, a roller chain drive system 110 includes adrive sprocket 112 and a driven sprocket 114 which incorporate thefeatures of the present invention therein. The roller chain drive system110 further includes a roller chain 116 having a number of rollers 118which engage and wrap about sprockets 112, 114. The roller chain rotatesin a clockwise direction as shown by arrow 11.

The roller chain 116 has two spans extending between the sprockets,slack strand 120 and taut strand 122. The roller chain 116 is undertension as shown by arrows 124. A central portion of the taut strand 122may be guided from the driven sprocket 114 to the drive sprocket 112with a chain guide 126. A first roller 128 is shown fully seated at a 12o'clock position on the drive sprocket 112. A second roller 130 isadjacent to the first roller 128 and is about to mesh with the drivesprocket 112.

To facilitate the description of an asymmetrical tooth profile of thepresent invention, reference will be made only to the drive sprocket112. However, the asymmetrical tooth profile of the present invention isequally applicable to the driven sprocket 114, as well as to idlersprockets and sprockets associated with counter rotating balance shafts.

Referring now to FIGS. 9 and 10, the sprocket 112 includes a first tooth132 having an engaging flank 134, and a second tooth 136 having a coastor disengaging flank 138. The engaging flank 134 and coast flank 138cooperate to define a tooth space 140 which receives the engaging roller128 (shown in phantom). The engaging roller 128 has a roller diameterD₁, and is shown fully seated in two-point contact within the toothspace 140 as described further below. More particularly, the engagingroller 128, when fully seated in the tooth space, contacts two lines Band C that extend axially along each sprocket tooth surface (i.e., in adirection orthogonal to the plane of the drawings). However, tofacilitate a description thereof, the lines A, B, and C are hereaftershown and referred to as contact points within the tooth space.

The engaging flank 134 has a radius R_(f) which is tangent to a radiallyouter end of a flank flat 144.

The location of the flank flat 144 is defined by an angle β, with theflat orientation being normal or perpendicular to a line that passesthrough Point B and the center of roller 128 when the roller iscontacting the sprocket at Points B and C. The length of the flank flatextending radially outward from Point B affects a time delay between aninitial tangential impact between sprocket 112 and roller 128 at a firstcontact Point A along the flank flat 144, and a subsequent radial impactat Point C. It is believed that the roller stays in contact with theflank flat from its initial tangential contact at Point A until theroller moves to a fully engaged two-point contact position at Points Band C. The pressure angle γ, the amount of pitch mismatch between thechain and the sprocket, and the length of the flank flat can be variedto achieve a desired initial roller contact Point A at the onset ofroller-sprocket meshing.

It should be appreciated that flank (tangential) contact always occursfirst, with radial contact then occurring always at Point C regardlessof chain pitch length. In contrast, with known tooth space forms (e.g.,ISO compliant and asymmetrical) incorporating single point contact (e.g.single line contact), an engaging roller must move to a driving positionafter making radial contact. The pressure angles γ therefore assume thatthe engaging roller will contact at the flank radius/root radiustangency point. Thus, the meshing contact location of the known singlepoint/line tooth space forms is pitch "sensitive" to determine where theradial impact as well as tangential impact will occur.

The engaging flank roller seating angle β (FIG. 9) and a disengagingflank roller seating angle β' replace the IS0-606 roller seating angle a(ISO profile shown in phantom). The pressure angle γ is a function ofthe engaging flank roller seating angle β. That is, as β increases, γdecreases. A minimum asymmetrical pressure angle can be determined fromthe following equation, where:

    γ.sub.min =β.sub.max -(α.sub.max /2+γ.sub.ISO min)

Therefore, an asymmetrical pressure angle γ_(min) =0 when β_(max)=(α_(max) /2+γ_(ISO) min) as illustrated in FIG. 28 FIG. 28 lists themaximum Beta (β) angles and the corresponding pressure angles (γ) forseveral sprocket sizes and several asymmetrical profiles. It should beappreciated that reducing the engaging flank pressure angle γ reducesthe tangential impact force component F_(IA) (FIG. 14) and thus thetangential impact noise contribution to the overall noise level at theonset of engagement.

Impact force F_(IA) is a function of the impact velocity which in turnis related to pressure angle γ. As pressure angle γ is reduced, itprovides a corresponding reduction in the impact velocity between thechain and the sprocket at the onset of meshing. A minimum pressure angleγ also facilitates a greater separation or distance between tangentialcontact points A and B to further increase or maximize engagement"staging". In the preferred embodiment, the engaging flank pressureangle γ is in the range of about -2.0° to about +5° to optimize thestaged impact between the roller and the sprocket.

In the embodiment being described, roller seating angle β is greaterthan ISO α_(max) /2 at a maximum material condition and β can beadjusted until a desired engaging flank pressure angle γ is achieved.For instance, the roller seating angle β of FIG. 9 provides a pressureangle γ that is less than zero, or a negative value. The negativepressure angle γ is best seen in FIG. 11, as contrasted with the ISO-606compliant tooth profile of FIG. 3 with a positive pressure angle γ. Asshown in FIG. 11, the asymmetrical profile pressure angle γ is definedas the angle between a line A extending from the center of the fullyengaged roller 128, when it is contacting the engaging tooth flank atpoints B and C, through point B, and a line B connecting the centers ofthe fully seated roller 128, and the center of the next meshing roller130 as if it were also two-point seated at full mesh in its engagingtooth space.

It is believed that a small negative pressure angle for the theoreticalchain/sprocket interface beneficially provides a pressure angle γ closerto zero (0) for a "nominal" system or for a system with wear. However,the engaging flank roller seating angle β may be beneficially adjustedso as to provide any engaging flank pressure angle γ having a value lessthan the minimum ISO-606 pressure angle.

Referring again to FIGS. 9 and 10, a first root radius R_(i) is tangentto a radially inner end of the flank flat 144, and tangent to a radiallyouter end of an inclined root surface 146. As best seen in FIG. 10, amaximum root radius R_(i) must be equal to, or less than, a minimumroller radius 0.5D₁ to facilitate the fully engaged two-point/linecontact at Points B and C. Accordingly, this will define a smallclearance 148 (FIG. 10) between the engaging flank 134 at root radiusR_(i) and roller 128 at full mesh (i.e., two-point/line contact). Theflank flat 144 and the inclined root surface 146 necessarily extendinside Points B and C respectively to facilitate the two-point/lineroller contact at full engagement for all dimensional toleranceconditions of the roller 128 diameter (D₁) and the root radius R_(i). Asecond root radius R_(i) ' is tangent to a radially inner end of theinclined root surface 146 at line 150. The coast flank has a radiusR_(f) ' with its arc center defined by disengaging side roller seatingangle β'.

The inclined root surface 146 is a flat surface having a finite lengthwhich defines a tooth space clearance (TSC). The tooth space clearancecompensates for chain pitch elongation or chain wear by accommodating aspecified degree of chain pitch elongation ΔP. In other words, the toothspace clearance TSC enables rollers of a worn chain to be maintained inhard contact with the inclined root surface of the sprocket teeth. Inaddition, the inclined root surface 146 facilitates reducing the radialreaction force thereby reducing the roller radial impact noisecontribution to the overall noise level.

The inclined root surface 146 may be inclined at any angle φ necessaryto satisfy a specific chain drive geometry and chain pitch elongation.As shown in FIG. 9, the inclined root surface angle φ is measured from aline 152 passing through the center of roller 128 and the sprocketcenter to a second line 154 which also passes through the center ofroller 128 and Point C. The inclined root surface 146 is normal to theline 154, and the inclined root surface extends radially inward to line150 where it is tangent to R_(i) '. In the embodiment being described,the inclined root surface angle φ is preferably in the range of about20° to about 35°.

FIG. 12 is an enlarged view of FIG. 8 showing the first roller 128 atfull engagement in two-point/line contact across the thickness or widthof the sprocket tooth profile, and the second roller 130 as the nextroller about to mesh with sprocket 112. As with the ISO compliant drivesystem 10, the chain guide 126 controls and guides a central portion thetaut strand 122 except for five unsupported link pitches extendingbetween the chain guide 126 and the engaging roller 128 (and except forthe unsupported link pitches extending between the driven sprocket andthe chain guide). The taut strand 122 is horizontal when roller 128 isat the 12 o'clock position.

FIG. 13 shows the drive sprocket 112 rotated in a clockwise direction(A/2)+ω, as determined by the instant of sprocket engagement by roller130. A straight line is assumed for the chain span from roller 128 to achain pin center 156, about which the unsupported span from pin center156 to engaging roller 130 is considered to rotate. It should beappreciated that the straight line assumption is valid only in aquasi-static model. The amount of movement (or deviation from thestraight line assumption) previously mentioned will be a function of thedrive dynamics as well as the drive and sprocket geometry.

The sprocket contact at the onset of mesh for roller 130 occurs earlierthan for the ISO counterpart, thereby reducing the amount of chordalrise and, just as importantly, allows the initial contact tobeneficially occur at a desired pressure angle γ on the engaging flankat Point A. Furthermore, the radial sprocket contact for roller 130,with its contribution to the overall noise level, does not occur untilthe sprocket rotation places roller 130 at the 12 o'clock position. Thisis referred to as staged engagement.

FIG. 14, an enlarged view of FIG. 13, more clearly shows the onset ofmeshing for roller 130. Just prior to the onset of mesh, roller 128 isassumed to carry the entire taut strand load F_(TB) +F.sub.φ, which loadis shown as force vector arrows. Actually, the arrows represent reactionforces to the taut strand chain force. At the instant of mesh for roller130, a tangential impact occurs as shown by impact force vector F_(IA).The tangential impact is not the same as the taut strand chain loading.In particular, impact loading or impact force is related to the impactvelocity V_(A). It is known that impact occurs during a collisionbetween two bodies, resulting in relatively large forces over acomparatively short interval of time. A radial impact force vectorF_(IC) is shown only as an outline in that the radial impact does notoccur until the sprocket has rotated sufficiently to place roller 130 ata 12 o'clock position.

FIG. 14a shows the same roller positions (solid) for rollers 128 and 130as shown in FIG. 14, but in addition, shows the roller positions (inphantom) relative to the sprocket profile once roller 130 reaches itstwo-point/line mesh at the 12 o'clock position. As a result of the pitchmismatch between the chain and sprocket, roller 128 must move to a newposition. In particular, as roller 130 moves from initial contact tofull mesh, roller 128 progresses forward in its tooth space. Smallclearances in the chain joints, however, reduce the amount of forwardprogression required for roller 128. Also occurring at the onset ofmeshing is the beginning of the taut strand load transfer from roller128 to roller 130.

The asymmetrical profile provides for the previously described "staged"meshing. In particular, referring again to FIG. 14, the Point Atangential contact occurs at the onset of mesh, with its related impactforce F_(IA). The roller 130 is believed to stay in hard contact withthe engaging flank 134 as the sprocket rotation moves the roller intofull mesh with its resulting radial contact at Point C. The radialimpact force F_(IC) (force vector shown as an outline) does not occuruntil the sprocket has rotated sufficiently to bring roller 130 intoradial contact at Point C.

FIG. 14b is an enlarged view of FIG. 14, except that sprocket 112 hasbeen rotated to advance roller 130 to the instant of full mesh at the 12o'clock position. At this instant of full mesh, the radial impact forceF_(IC) occurs and the taut strand load transfer is considered to becomplete. At the instant of the radial collision by roller 130 at PointC, with its resultant radial impact force F_(IC), the tangential impactforce of F_(IA) has already occurred and is no longer a factor. The timedelay ("staged" engagement) between the tangential and radialroller-sprocket collisions effectively spreads the roller sprocketmeshing impact energy over a greater time interval, thereby reducing itscontribution to the generated noise level at mesh frequency.Additionally, it is believed that the present asymmetrical sprockettooth profile beneficially permits a more gradual taut strand loadtransfer from a fully engaged roller 128 to a meshing roller 130 as themeshing roller 130 moves from its Point A initial mesh to its fulltwo-point mesh position.

Referring again to FIG. 14, the chordal rise (and fall) with the presentasymmetrical profile is the perpendicular displacement of the center ofroller 130 from the taut strand 122 path as it moves from its initialmeshing contact Point A to the mesh position presently occupied byroller 128. It is believed that roller 130 will stay in hard contactwith the engaging flank 134 as the roller moves from initial tangentialcontact to full mesh, and accordingly, the chordal rise is reduced asthe distance between Points A and B is increased. As shown in FIG. 14,chain pitch P_(C) is beneficially greater than sprocket 112 chordalpitch P_(S).

Referring now to FIG. 15, the length of the inclined root surface 146(FIG. 10) may be reduced to zero (0), thereby eliminating the inclinedroot surface 146 and permitting root radius R_(i) ' to be tangent to theroot surface and the roller 142 at Point C. That is, R_(i) ' is tangentto a short flat at Point C, and the flat is tangent to R_(i). If theinclined root surface 146 is eliminated, the engaging flank pressureangle γ would generally be in the range of some positive value to zero,but normally not less than zero. The reason is that a negative γrequires chordal pitch reduction so that the roller 60 can exit thesprocket wrap 60 (see FIGS. 16 and 17) without interfering with R_(f).

FIG. 16 shows the roller contact to the sprocket 112 profile for all therollers in the wrap 60. Roller 128 is in full two-point mesh as shown.Line 160 shows the contact point for each of the rollers, as well as thecontact progression as the rollers travel around the wrap. The inherentpitch mismatch between the sprocket and roller chain causes the rollersto climb up the coast side flank as the rollers progress around thesprocket wrap. With the addition of appreciable chordal pitch reduction,the extent to which the rollers climb up the coast side flank inincreased.

It is important to note that chordal pitch reduction is required whenthe pressure angle γ has a negative value. Otherwise, as shown in FIGS.16 and 17, roller 162 would interfere with the engaging flank (with amaximum material sprocket and a theoretical pitch [shortest] chain) asit exits the wrap 60 back into the span. Also, the reduced chordal pitchassists the staged mesh as previously mentioned. FIG. 16, showing theroller contact progression in the wrap 60, serves also to show why theshallow β' angle and tooth space clearance TSC helps maintain "hard"roller-sprocket contact for the rollers in the wrap.

In addition, the disengaging flank roller seating angle β' (FIG. 9) maybe adjusted to have a maximum value which is equal to α_(min) /2 or evenless. This reduced seating angle β' promotes faster separation when theroller leaves the sprocket and enters the span. This reduced angle β'also allows for the roller in a worn chain to ride up the coast flanksurface to a less severe angle as the roller moves around the sprocketin the wrap. Accordingly, chordal pitch reduction, if used in thisembodiment, should be a small value.

It is contemplated that the above-described asymmetrical tooth profilefeatures can be altered without substantially deviating from the chainand sprocket meshing kinematics that produce the noise reductionadvantages of the present invention. For example, the engagingasymmetrical flank profile could be approximated by an involute form,and the disengaging asymmetrical flank profile could be approximated bya different involute form. Slight changes to the asymmetrical toothprofiles may be made for manufacturing and/or quality control reasons,or simply to improve part dimensioning. These changes are within thescope of the invention as disclosed herein.

In a further embodiment, the engaging flank inclined root surface 146(FIG. 9) may be replaced with a coast flank inclined root surface 164 asshown in FIG. 18. The coast flank inclined root surface 164 providestooth space clearance (TSC) in the same manner as described above withregard to the inclined root surface 146. In addition, the disengagingflank inclined root surface 164 beneficially moves the roller to apreferred radially outward position as the chain wears.

Alternatively, the disengaging flank inclined root surface 164 may beincluded with the engaging flank inclined root surface 146 as shown inFIG. 19. The engaging flank and disengaging flank inclined root surfaces146, 164 cooperate to provide tooth space clearance (TSC) in the samemanner as previously described.

Referring now to FIG. 20, any one of the above-described asymmetricaltooth profile embodiments of FIGS. 9, 15, 18, and 19 may be incorporatedinto a random-engagement roller chain sprocket 300. The sprocket 300 isshown as an 18-tooth sprocket. However, the sprocket 300 may have moreor less teeth, as desired. The sprocket 300 includes a first group ofarbitrarily positioned sprocket teeth 302 each having a profile whichincorporates the flank flat 144 shown in FIGS. 9, 15, 18 and 19. Inorder to facilitate describing the random engagement roller chainsprocket, reference will be made to the sprocket teeth 302 having thetooth profiles shown in FIGS. 9 and 15. However, the sprocket teeth 302may equally incorporate the tooth profiles shown in FIGS. 18 and 19. Theremaining sprocket teeth 304 (sprocket teeth 1, 3, 4, 9, 13, 14, and 16)are randomly positioned around the sprocket and incorporate a toothprofile different from that of the first group of sprocket teeth 302. Asdescribed further below, the first and second groups of sprocket teeth302, 304 cooperate to reduce chain drive system noise levels below anoise level which either tooth profile used alone would produce.

FIG. 21 illustrates an exemplary tooth profile for one of the sprocketteeth 304. An engaging flank 306 and a coast or disengaging flank 308 ofan adjacent tooth cooperate to define a tooth space 310 which receivesan engaging roller 314 (shown in phantom). The engaging roller 314 has aroller diameter D_(i), and is shown fully seated in two-point contactwithin the tooth space 310. The engaging roller 314 initially contactsthe engaging flank 306 at point A' before fully seating in the toothspace at points B' and C. Contact points B' and C are actually linesthat extend axially along each sprocket tooth surface (i.e., in adirection orthogonal to the plane of the drawings).

The engaging flank 306 has a radius R_(f) which is tangent to a flatsurface (not shown) at contact point B'. The flat surface, whichfunctions only to facilitate the two-point roller contact (describedfurther below), is normal to the roller 314 at point B'. The flatextends radially inward from point B' and is tangent to a first rootradius R_(i). The first root radius R_(i) may be tangent to a radiallyouter end of an inclined root surface 316. A maximum root radius R_(i)must be equal to, or less than, a minimum roller radius to facilitatethe fully engaged two-point/line contact at points B' and C.Accordingly, a small clearance is defined between the root radius R_(i)and the roller 314 when the roller 314 is fully seated at points B' andC. The flat surface and the inclined root surface 316 necessarily extendinside contact points B' and C, respectively, to facilitate thetwo-point/line roller contact at engagement. A second root radius R_(i)' is tangent to a radially inner end of the inclined root surface 316 atline 318. The disengaging flank has a radius R_(f) ' at a point definedby the roller seating angle β'.

The inclined root surface 316 is a flat surface having a finite lengthwhich defines a tooth space clearance (TSC). The tooth space clearancecompensates for chain pitch elongation or chain wear by accommodating aspecified degree of chain pitch elongation. In other words, the toothspace clearance TSC enables rollers of a worn chain to be maintained inhard contact with the inclined root surface of the sprocket teeth. Inaddition, the inclined root surface 316 facilitates reducing the radialreaction force thereby reducing the roller radial impact noisecontribution to the overall noise level.

The inclined root surface 316 may be inclined at any angle φ necessaryto satisfy a specific chain drive geometry and chain pitch elongation.The inclined root surface angle φ is measured from a line 320 passingthrough the center of roller 314 and the sprocket center to a secondline 322 which also passes through the center of roller 314 and throughcontact point C. The inclined root surface 316 is normal to the line322, and the inclined root surface extends radially inward to line 318where it is tangent to R_(i) '. In the embodiment being described, theinclined root surface angle φ is preferably in the range of about 20° toabout 35°. In sum, the tooth profile for the sprocket teeth 304 may besubstantially the same as the tooth profile shown in FIGS. 9 and 15except that the tooth profile 304 does not have an engaging flank flatextending above (i.e. radially outward of) contact point B'.

As with the tooth profile of FIGS. 9 and 15, the engaging flank inclinedroot surface 316 may be replaced with a disengaging flank inclined rootsurface as in FIG. 18. That is, the tooth profile 304 may besubstantially identical to the sprocket 112 shown in FIG. 18 fromcontact point C to the outer diameter of the disengaging flank 308. Thedisengaging flank inclined root surface provides tooth space clearance(TSC) in the same manner as the inclined root surface 316. In addition,the coast flank inclined root surface beneficially moves the roller to apreferred radially outward position as the chain wears. Alternatively,the disengaging flank inclined root surface may be included with theengaging flank inclined root surface 316 as in FIG. 19. Thus, the toothprofile 304 may be substantially identical to the sprocket 112 shown inFIG. 19 from contact point C to the outer diameter of the coast flank138.

Pitch mismatch is inherent in a chain/sprocket interface except at onecondition--the theoretical condition which is defined as a chain at itsshortest pitch (shortest being theoretical pitch) and a maximum materialsprocket. This theoretical condition therefore defines one limit (zero,or no pitch mismatch) of the tolerance range of the pitch mismatchrelationship of chain and sprocket. The other limit is defined when alongest "as built" chain is used with a sprocket at minimum materialconditions--or in other words, a sprocket having a minimum profile. Thislimit produces the greatest amount of pitch mismatch. The pitch mismatchrange is therefore determined by the part feature tolerances.

Additional pitch mismatch may be introduced to facilitate a greater timedelay between tangential contact at point A (for tooth profile 302) andtangential contact at point A' (for tooth profile 304). That is, varyingthe time at which roller-to-sprocket contact occurs for each toothprofile 302, 304 results in reduced mesh frequency noise because thepoint and rhythm of the initial roller-to-sprocket contact is altered.The time delay between the roller-to-sprocket contact at points A and A'may be increased by increasing the mismatch between the chain pitch andsprocket pitch.

Additional pitch mismatch may also be introduced to facilitate a"staged" roller contact for each tooth profile 302, 304. That is,additional pitch mismatch increases the time delay between initialtangential contact and the fully seated radial contact for each toothprofile 302, 304. It should be appreciated that staged contact isgreater for the tooth profile 302 than for the tooth profile 304 due tothe flank flat 144 which causes initial contact to occur higher up onthe engaging flank of the sprocket teeth 302.

The sprocket chordal pitch is necessarily shorter than the chain pitchto facilitate the "staged" roller-tooth contact. In addition, chordalpitch reduction also provides roller-to-flank clearance as the rollerexits the sprocket wrap back into the strand. Added chordal pitchreduction, when used, is preferably in the range of 0.005-0.030 mm.

The staged roller contact for each tooth profile 302, 304 may be furtherassisted by providing sprocket tooth pressure angles γ that aresubstantially less than the ISO standard. Pressure angles γ equal to orvery close to zero (0), or even negative pressure angles, arecontemplated. For instance, FIG. 22 illustrates one embodiment of arandom engagement sprocket 330 wherein the tooth profiles 302, 304 havethe same, or at least substantially the same, pressure angles γ (thus,the profiles 302, 304 have the same or at least substantially the sameroller seating angles β₃₀₂ and β₃₀₄). In FIG. 22, the pressure angle forboth profiles 302, 304 is zero or a positive value. Thus, γ_(min) forboth profiles 302, 304 may be 0°, and γ_(max) for both profiles 302, 304is equal to some value less that the ISO minimum pressure angle γ. As aresult, initial roller-to-sprocket contact occurs at point A' followedby subsequent radial contact at points B and C for the tooth profile 302of sprocket 330. And, initial roller-to-sprocket contact occurs at pointA' followed by subsequent radial contact at points B and C for the toothprofile 304 of sprocket 330. The sprocket 330 may, or may notincorporate additional chordal pitch reduction, and may, or may notincorporate tooth space clearance (TSC), as described above.

With reference to FIG. 23, a second embodiment of a random engagementsprocket 340 is shown wherein the pressure angles γ for the toothprofiles 302, 304 are different. That is, both pressure angles γ areeither a positive value or zero, but the pressure angle γ for toothprofile 302 is smaller than the pressure angle γ for tooth profile 304(and the roller seating angle β₃₀₂ is greater than the roller seatingangle β₃₀₄). Thus, γ_(min) may be equal to 0° for the tooth profile 302,γ_(min) may be equal to +4° for the tooth profile 304, and γ_(max) forboth profiles 302, 304 will always be less than the ISO minimum pressureangle (the feature tolerance band or range for γ_(min) and γ_(max) isthe same for both tooth profiles 302, 304). As a result, initialroller-to-sprocket contact occurs at point A followed by subsequentradial contact at points B and C for the tooth profile 302 of sprocket340. And, initial roller-to-sprocket contact occurs at point A' followedby subsequent radial contact at points B' and C for the tooth profile304 of sprocket 340. The sprocket 340 may or may not incorporateadditional chordal pitch reduction, and may or may not incorporate toothspace clearance (TSC), as desired.

Alternatively, the pressure angle γ_(min) for the profile 302 may alwaysbe a negative value, while the pressure angle γ_(min) for profile 304may always be a positive value or zero. For instance, γ_(min) may beequal to -3° for the tooth profile 302, γ_(min) may be equal to +3° forthe tooth profile 304, and γ_(max) for both profiles 302, 304 willalways be less than the ISO minimum pressure angle. With thisembodiment, additional chordal pitch reduction will always be included,however, tooth space clearance may or may not be included.

FIG. 24 shows the engagement path of a roller 342 fully seated in thesprocket tooth 302, and an engagement path of the roller 314 engagingthe adjacent sprocket tooth 304 of the sprocket 340. At the instant ofinitial contact for roller 314, the chain load transfer from roller 342to roller 314 begins, until at full engagement, roller 314 willsubstantially carry the full chain load. Also, as roller 314 moves frominitial contact to full mesh, roller 342 will progress forward in itstooth space. Small clearances in the chain joints, however, will reducethe amount of forward progression required for Roller 342. Since impactenergy is reduced with this profile geometry, the load transfer is moregradual than with the standard ISO profile.

The references l_(A) and l_(B) in FIG. 24 serve to show the amount of"staging" for each profile from the initial contact to the two-pointcontact at full mesh. The reference l_(A) indicates the linear distanceseparating the initial contact point A from the contact point B for theprofile 302, and the reference l_(B) indicates the linear distanceseparating the initial contact point A' from the contact point B' forthe profile 304. The value l_(A) -l_(B) is a measure of the initial(tangential) contact time delay from profile 304 to profile 302, whichis illustrated further in FIGS. 25 and 26. The time delay beneficiallyincreases with chain wear for the embodiment incorporating differentpressure angles (FIG. 23) in that the tooth profile 304 "falls away"faster than does the tooth profile 302. The delay increases withincreasing chain pitch or pitch mismatch.

FIGS. 25 and 26 illustrate the meshing delay between the tooth profiles302, 304. In particular, as shown in FIG. 25, the sprocket 340 has aroller 344 fully-seated in two-point contact with a sprocket toothincorporating the tooth profile 302. The roller 342 is shown at theinstant of initial tangential contact at point A of a second sprockettooth also incorporating the tooth profile 302. The roller 314 is thenext roller in the span and will mesh with a sprocket toothincorporating the tooth profile 304. The sprocket 340 must rotatethrough an angle τ for roller 342 to move from its initial contactposition at point A to full mesh, seated in two-point contact with thetooth profile 302 at a 12 o'clock position.

With reference now to FIG. 26, the sprocket 340 is shown rotated in aclockwise direction until roller 314 is at the instant of initialtangential contact at point A' of the tooth profile 304. The sprocket340 must now rotate through a smaller angle κ for roller 314 to fullyseat in two-point contact with the tooth profile 304 at a 12 o'clockposition. That is, the staged contact for the tooth profile 302 isgreater than for the tooth profile 304 due to the flank flat 144 as wellas the difference in engaging side roller seating angles β₃₀₂ and β₃₀₄which cause initial contact to occur higher up on the engaging flank ofthe sprocket teeth 302. Thus, the sprocket 340 must rotate through anadditional angle τ-κ in order for the roller 314 to fully seat.

As shown in FIG. 20, the two sets of tooth profiles 302, 304 arearranged in a random pattern in order to modify the meshing impactfrequency by altering the point and rhythm of initial roller-to-sprocketcontact. However, the two sets of tooth profiles 302, 304 could bearranged in many different random patterns. Further, it is alsocontemplated that the two sets of tooth profiles 302, 304 could bearranged in many regular patterns that would work equally as well. Inall cases, the arrangement of two sets of different tooth profiles on asprocket provides a means for breaking up the mesh frequency impactnoise normally associated with and induced by a full complement ofsubstantially identically shaped sprocket teeth. The mesh frequencynoise reduction is achieved by altering the point and rhythm of initialroller-to-sprocket contact.

The crankshaft sprocket, generally the smallest sprocket in the chaindrive, is usually the major noise contributor. The typically largerdriven camshaft sprocket, however, will also contribute to the generatednoise levels, but generally to a lesser extent than the crankshaftsprocket. However, the driven sprocket, particularly if it is nearly thesame size or smaller than the driving sprocket, may be the prime noisegenerator, as in the case with balance shaft sprockets and pumpsprockets. Thus, the features of the present invention may also be usedadvantageously with camshaft or driven sprockets as well.

It should be appreciated that the tooth profile features of FIGS. 20-26can be altered slightly without substantially deviating from the chainand sprocket meshing kinematics that produce the noise reductionadvantages of the present invention. For example, the engagingasymmetrical flank profile could be approximated by an involute form,and the disengaging asymmetrical flank profile could be approximated bya different involute form. Slight changes to the profile may be done formanufacturing and/or quality control reasons--or simply to improve partdimensioning.

The invention has been described with reference to the preferredembodiments. Obviously, modifications will occur to others upon areading and understanding of this specification and this invention isintended to include same insofar as they come within the scope of theappended claims or the equivalents thereof.

Having thus described the preferred embodiments, the invention is now claimed to be:
 1. A sprocket comprising:a first plurality of sprocket teeth; a second plurality of sprocket teeth; a first one of said first plurality of sprocket teeth cooperating with a second one of said first sprocket teeth or with a first one of said second sprocket teeth to define a first tooth space including a first engaging flank, a first flat surface tangent to and extending radially inward of said first engaging flank, a first root surface portion tangent to and extending radially inward of said first flat surface, and a second flat surface tangent to and extending radially inward of said first root surface portion, said first flat surface having a first portion and a second portion, and said second flat surface having a third portion, whereby said first portion facilitates staging a first meshing between an associated roller and the sprocket, and said second portion and said third portion facilitate an associated roller seating in two-point contact within said first tooth space; and a second one of said second plurality of sprocket teeth cooperating with a third one of said first sprocket teeth or with a third one of said second sprocket teeth to define a second tooth space including a second engaging flank, a third flat surface tangent to and extending radially inward of said second engaging flank, a second root surface portion tangent to and extending radially inward of said third flat surface, and a fourth flat surface tangent to and extending radially inward of said second root surface, said third flat surface and said fourth flat surface facilitating an associated roller seating in two-point contact within said second tooth space.
 2. The sprocket of claim 1, wherein said first meshing includes an initial tangential impact along said first flat surface followed in time by a subsequent radial impact.
 3. The sprocket of claim 1, wherein an engaging flank pressure angle (γ) of said first plurality of sprocket teeth is substantially the same as an engaging flank pressure angle (γ) of said second plurality of sprocket teeth.
 4. The sprocket of claim 1, wherein a minimum engaging flank pressure angle (γ_(min)) of said first and said second plurality of sprocket teeth is greater than or equal to zero, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 5. The sprocket of claim 1, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is greater than or equal to zero, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is greater than said minimum engaging flank pressure angle of said first plurality of sprocket teeth, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 6. The sprocket of claim 1, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is a negative value, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is a positive value, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 7. The sprocket of claim 1, wherein the first tooth space and the second tooth space both have an asymmetrical tooth profile defined by an engaging side roller seating angle (β) that is greater than a disengaging side roller seating angle (β').
 8. A sprocket comprising:a first plurality of sprocket teeth; a second plurality of sprocket teeth; a first one of said first plurality of sprocket teeth cooperating with a second one of said first sprocket teeth or with a first one of said second sprocket teeth to define a first tooth space including a first engaging flank, a first flat surface tangent to and extending radially inward of said first engaging flank, a first root surface portion tangent to and extending radially inward of said first flat surface, and a second flat surface tangent to and extending radially inward of said first root surface portion, a first radius defining said first root surface portion being less than a radius of an associated roller so that a first clearance exists between said first root surface portion and an associated roller when an associated roller is seated within said toothspace; and a second one of said second plurality of sprocket teeth cooperating with a third one of said first sprocket teeth or with a third one of said second sprocket teeth to define a second tooth space including a second engaging flank, a third flat surface tangent to and extending radially inward of said second engaging flank, a second root surface portion tangent to and extending radially inward of said third flat surface, and a fourth flat surface tangent to and extending radially inward of said second root surface, a second radius defining said second root surface portion being less than a radius of an associated roller so that a second clearance exists between said second root surface portion and an associated roller when an associated roller is seated within said second tooth space.
 9. The sprocket of claim 8, wherein an engaging flank pressure angle (γ) of said first plurality of sprocket teeth is substantially the same as an engaging flank pressure angle (γ) of said second plurality of sprocket teeth.
 10. The sprocket of claim 8, wherein a minimum engaging flank pressure angle (γ_(min)) of said first and said second plurality of sprocket teeth is greater than or equal to zero, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 11. The sprocket of claim 8, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is greater than or equal to zero, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is greater than said minimum engaging flank pressure angle of said first plurality of sprocket teeth, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 12. The sprocket of claim 8, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is a negative value, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is a positive value, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 13. The sprocket of claim 8, wherein the first tooth space and the second tooth space both have an asymmetrical tooth profile defined by an engaging side roller seating angle (β) that is greater than a disengaging side roller seating angle (β').
 14. A sprocket comprising:a first plurality of sprocket teeth; a second plurality of sprocket teeth; a first one of said first plurality of sprocket teeth cooperating with a second one of said first sprocket teeth or with a first one of said second sprocket teeth to define a first tooth space including a first engaging flank, a first flat surface tangent to and extending radially inward of said first engaging flank, a first root surface portion tangent to and extending radially inward of said first flat surface, and a second flat surface tangent to and extending radially inward of said first root surface portion; a second one of said second plurality of sprocket teeth cooperating with a third one of said first sprocket teeth or with a third one of said second sprocket teeth to define a second tooth space including a second engaging flank, a third flat surface tangent to and extending radially inward of said second engaging flank, a second root surface portion tangent to and extending radially inward of said third flat surface, and a fourth flat surface tangent to and extending radially inward of said second root surface; and the sprocket being adapted for staged meshing with an associated roller chain having a plurality of rollers wherein a first roller of the associated roller chain impacts at a first contact point along the first engaging flank and a second roller of the associated roller chain impacts at a second contact point along the second engaging flank, and first contact point is spaced radially outward from the second contact point.
 15. The sprocket of claim 14, wherein said first flat surface includes a first portion and a second portion and said second flat surface includes a third portion whereby said first portion facilitates locating the first contact point radially outward of the second contact point and said second portion and said third portion facilitate seating the associated rollers in two-point contact within said respective tooth spaces.
 16. The sprocket of claim 14, wherein an engaging flank pressure angle (γ) of said first plurality of sprocket teeth is substantially the same as an engaging flank pressure angle (γ) of said second plurality of sprocket teeth.
 17. The sprocket of claim 14, wherein a minimum engaging flank pressure angle (γ_(min)) of said first and said second plurality of sprocket teeth is greater than or equal to zero, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 18. The sprocket of claim 14, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is greater than or equal to zero, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is greater than said minimum engaging flank pressure angle of said first plurality of sprocket teeth, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 19. The sprocket of claim 14, wherein:a minimum engaging flank pressure angle (γ_(min)) of said first plurality of sprocket teeth is a negative value, a minimum engaging flank pressure angle (γ_(min)) of said second plurality of sprocket teeth is a positive value, and a maximum engaging flank pressure angle (γ_(max)) of said first and said second plurality of sprocket teeth is less than an ISO-606 minimum engaging flank pressure angle (γ_(ISOmin)).
 20. The sprocket of claim 14, wherein the first tooth space and the second tooth space both have an asymmetrical tooth profile defined by an engaging side roller seating angle (β) that is greater than a disengaging side roller seating angle (β'). 